WIP: Resolve "Homogeneize formatting in the C++ code"

What does this MR do?

Applies clang-format and updates our contribution guide.

Is there something that needs to be double checked?

Are other branches strongly affected by this change?

Can this MR be accepted?

• Implemented
  • Added .clang-format file
  • clang-format is applied
  • Updated contribution guilde
  • Added git hook to help application of the format
  • Update gitlab template with format checkbox
• Documentation is not chenged
• Pipeline passing
• Squash option set
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #180 (closed)